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Objective: We aimed to assess the following: (1) the frequency of axial spondyloarthritis (axSpA) according to extra‐articular presentation and HLA‐B27 status, (2) clinical and imaging features that distinguish axSpA from non‐axSpA, and (3) the impact of magnetic resonance imaging (MRI) on diagnosis and classification of axSpA. Methods: The Screening for Axial Spondyloarthritis in Psoriasis, Iritis, and Colitis (SASPIC) study enrolled patients in two multicenter cohorts. Consecutive patients with undiagnosed chronic back pain attending dermatology, ophthalmology, and gastroenterology clinics with psoriasis (PsO), acute anterior uveitis (AAU), or inflammatory bowel disease (IBD) were referred to a local rheumatologist with special expertise in axSpA for a structured diagnostic evaluation. The primary outcome was the proportion of patients diagnosed with axSpA by the final global evaluation. Results: Frequency of axSpA was 46.7%, 61.6%, and 46.8% in patients in SASPIC‐1 (n = 212) and 23.5%, 57.9%, and 23.3% in patients in SASPIC‐2 (n = 151) with PsO, AAU, or IBD, respectively. Among those who were B27 positive, axSpA was diagnosed in 70%, 74.5%, and 66.7% of patients in SASPIC‐1 and in 71.4%, 87.8%, and 55.6% of patients in SASPIC‐2 with PsO, AAU, or IBD, respectively. All musculoskeletal clinical features were nondiscriminatory. MRI was indicative of axSpA in 60% to 80% of patients and MRI in all patients (SASPIC‐2) versus on‐demand (SASPIC‐1) led to 25% fewer diagnoses of axSpA in patients who were HLA‐B27 negative with PsO or IBD. Performance of the Assessment of SpondyloArthritis International Society classification criteria was greater with routine MRI (SASPIC‐2), though sensitivity was lower than previously reported. Conclusion: Optimal management of patients presenting with PsO, AAU, IBD, and undiagnosed chronic back pain should include referral to a rheumatologist. Conducting MRI in all patients enhances diagnostic accuracy. [ABSTRACT FROM AUTHOR]